1. Field of the Invention
The present invention relates to an ophthalmologic apparatus, a method for controlling the ophthalmologic apparatus, and a storage medium storing instructions for controlling the ophthalmologic apparatus.
2. Description of the Related Art
A refraction test is an eye exam technique to determine whether a person's eye has a refractive error. Thus, the refraction test enables an ophthalmologist or optometrist to measure a person's prescription for eyeglasses or contact lenses. Fogging is a technique widely used in conventional eye refractive power measurement apparatuses which measure information about refractive power of a subject's eye. Fogging includes projecting a fixation target onto the subject's eye and moving the position of the fixation target image from a reference position based on a result of preliminary measurement to a far side, thereby promoting relaxation of the subject's eye.
If the subject's eye has a high degree of astigmatism, the subject's eye may fail to relax sufficiently. Japanese Patent Application Laid-Open No. 06-233740 discusses a technology for performing feedback control to move the position of the fixation target image in a direction of relaxing the accommodation ability according to measured astigmatism information.
Elderly people may have a pupil diameter of 2 mm or less. To prevent a measurement failure due to shielding of a light flux by the iris, Japanese Patent No. 4233426 discusses a technique for providing a ring-shaped aperture to be conjugate with a small pupil aside from one for a standard pupil.
In order for the subject's eye to be sufficiently relaxed by fogging control, the subject's eye needs to visually identify a change of the image forming position of the fixation target image. If the subject's eye has a small pupil diameter and thus a large depth of focus, a change of the image forming position is less visually identifiable. This has resulted in a problem that the eye having a small pupil diameter fails to be sufficiently relaxed and the eye refractive power measurement ends without a change in measurement.
More specifically, a subject's eye having a standard pupil can visually identify a change of the fixation target image when the fixation target image is moved to a far side. Measurement is successively performed with the subject's eye retaining a less and less accommodation ability, so that the measurement can be ended with no accommodation ability. On the other hand, the eye having a small pupil diameter may fail to visually identify a change of the fixation target image even if the fixation target image is moved to a far side. The accommodation ability remains unchanged, and measurement is successively performed with the accommodation ability still remaining. As a result, the measurement may be ended because there is no change in measurements.